1. Field of the Invention
This invention relates to exhaust nozzles and particularly to a new and improved exhaust nozzle in which the size and shape of the exhaust opening can be varied.
2. Description of the Prior Art
The efficiency of a gas turbine engine can be improved through the use of an exhaust nozzle in which the size and shape of the exhaust opening can be varied in response to changes in operating conditions, such as, in the case of an aircraft engine, changes in airspeed and altitude. In the case of a two-dimensional convergent-divergent exhaust nozzle, i.e., a 2D-CD nozzle, varying the size and shape of the exhaust opening can present particular problems. By a "two-dimensional" nozzle it is meant that the nozzle has a generally rectangular exhaust opening with the variable geometry taking place in a plane and not affecting the third dimension. By "convergent-divergent" it is meant that in the downstream direction, the area of the exhaust opening of the nozzle first decreases to a position of minimum area, called the throat, and then increases.
An approach to varying the size and shape of an aircraft 2D-CD nozzle is to utilize a plurality of moveable flaps to define the upper and lower boundaries of the exhaust opening. Such flaps, however, are subject to heavy pressure loads from the exhaust gases. The actuation mechanisms which move the flaps must be capable of working under such pressure loads and are thus often relatively large and heavy. The extra size of the mechanism can cause an increase in drag and the extra weight requires additional thrust to support the weight. Thus, large and heavy actuation mechanisms for 2D-CD nozzles often reduce the engine efficiency for which the nozzle was designed.
Another problem encountered with upper and lower moveable flaps in a 2D-CD nozzle is in the synchronization of the actuation mechanisms. In order to maintain the direction of engine thrust along the engine centerline, all movements of the upper and lower flaps must be synchronized such that they are equal in magnitude but opposite in direction. Where there are separate actuation mechanisms for the upper and lower nozzle flaps, synchronization means must be included to ensure synchronized movement between the flaps. Such synchronization means not only can increase nozzle weight and size, but can also increase the production cost of the nozzle.
Another problem encountered in the use of 2D-CD nozzles is that it is difficult to fabricate a common nozzle flap actuation mechanism which can be used on different aircraft/engine combinations. One reason for this is that for a specific aircraft/engine combination, there exists a schedule such that for each value of nozzle throat area, there is a corresponding optimum value of nozzle discharge area to obtain maximum engine efficiency. Since each actuation mechanism is fabricated so as to operate according to a certain throat area/discharge area schedule, it can only be used on an aircraft/engine combination which utilizes that schedule. To change the schedule of an actuation mechanism which uses only mechanical link arms to effect movement of the nozzle flaps requires replacement of the link arms with longer or shorter ones. Such replacement is time-consuming and expensive, involving a redesign of the entire actuation mechanism.
In view of the above-noted problems, it is, therefore, an object of the present invention to provide an improved exhaust nozzle having flap actuation means which are compact in size and weight and which thus promote engine efficiency.
Another object of the present invention is to provide an exhaust nozzle in which no synchronization is required between flap actuation means.
Yet another object of the present invention is to provide an exhaust nozzle in which the throat area/discharge area schedule of the actuation means can be simply and inexpensively modified.